Everyone knows that secondhand smoke has harmful effects for children. Yet until now, traffic pollution has largely remained under the radar when it comes to the study of children's health. New research, though, shows that 14 percent of chronic childhood asthma is due to exposure to traffic pollution near busy roads.

The research, which was conducted in 10 European cities, applied a statistical technique known as "population-attributable fractions" to existing data in order to learn how much childhood asthma can be blamed on heavy traffic. The technique calculates the proportional reduction in disease or death that would occur if exposure to a risk factor, such as traffic pollution, were reduced to a lower level. The data itself was taken from existing epidemiological studies which found that children exposed to traffic pollution also had higher rates of asthma. In order to make sure that their study wasn't swayed by other factors, the researchers made sure to rule out other contributing influences such as chain-smoking parents and socioeconomic status.

In the end, the researchers found that the results were comparable to the burden associated with secondhand smoke--a major contributer of asthma in children. The World Health Organization estimates that between 4 to 18 percent of asthma cases are linked to this type of smoke.

The study shows that traffic pollution can vastly impact children who live in urban areas. The lead author of the study, Laura Perez, said in an interview with Science Codex, "Air pollution has previously been seen to trigger symptoms, but this is the first time we have estimated the percentage of cases that might not have occurred if Europeans had not been exposed to road traffic pollution. In light of all of the existing epidemiological studies showing that road-traffic contributes to the onset of the disease in children, we must consider these results to improve policy making and urban planning."